The present invention relates to a method of fabricating multi-mode optical fiber preforms by the VAD (Vapor-phase Axial Deposition) method. Reference is also made to copending application Ser. No. 188,914 filed by Kawachi et al on Sept. 19, 1980 and which relates to a method of fabricating single-mode optical fiber preforms.
The process for fabricating optical fiber preforms by the VAD method is disclosed in U.S. Pat. No. 4,062,665. In the VAD method, fine glass particles, synthesized by hydrolysis or the thermal oxidation reaction of glass raw material with a flame, are attached to and deposited on one end of a seed rod in the axial direction so as to form a cylindrical porous preform. The porous preform is heated at a high temperature and vitrified into a transparent preform.
In the conventional VAD method for fabricating a cylindrical porous glass preform, a glass synthesizing torch is disposed on or in parallel with the axis of rotation of the porous glass preform. Further, an exhaust nozzle for removing residual glass particles not attached to the growing surface of the porous glass preform is disposed on the side of a reaction vessel. In this case, the growing speed of the porous preform in the axial direction is likely to be slow and in an extreme case the growing speed is higher in the radial direction than in the axial direction. The residual glass particles are additionally attached to the periphery of the upper porous preform so that the outer diameter of the preform thus obtained greatly fluctuates. Because of this disadvantage of the conventional VAD method, it has been very difficult to improve the transmission bandwidth properties of a multi-mode optical fiber by controlling an obtained graded-type refractive index profile by adjusting the dopant concentration in the radial direction of the glass preform and to improve transmission loss properties by the simultaneous formation of core and cladding regions. For this reason, the conventional VAD method fails to take full advantage of the feasibility of mass-production of optical fibers which is a feature of the VAD method.
A different construction has been proposed in Electronics Letters, Aug. 17, 1978, Vol. 14, No. 17, pp. 534-535, by S. Sudo et al. In this construction, a main torch and a subsidiary torch are provided, the main torch being disposed on the rotation axis of the porous preform while the subsidary torch is inclined to the rotation axis. With this arrangement of torches, the glass particles from the subsidiary torch are deposited on the peripheral portion of the porous glass preform in a manner such that the refraction index profile in the radial direction of the preform is adjustable. This proposal, however, still involves the disadvantage of an undesirable outer diameter fluctuation of the porous glass preform making it difficult to stably manufacture long-length optical fibers by the VAD method.
Additionally, in those cases where the amount of residual glass particles is fairly large, a glass particle layer with a small apparent density is formed by the residual glass particles on the side wall of the porous preform. Accordingly, the outer diameter of the porous glass preform is remarkably large or "cracking" may be formed on the peripheral wall of the porous glass preform. As a result, after the preform is vitrified to obtain a transparent preform, it is difficult to use the vitrified preform as an optical fiber preform.